The invention is based on a priority application EP 00440249.1, which is hereby incorporated by reference.
The invention is related to a method for monitoring an optical waveguide between a monitoring terminal point and a monitoring unit, in which an optical data signal from a transmitter, and for amplification, pump light from a pump laser, is emitted into the optical waveguide.
In modern optical transmission systems, optical waveguides are being increasingly employed over long routes without opto-electrical conversion and intermediate electrical amplification of the transmitted data signal. In that case the only active system components are optical amplifiers which compensate only the attenuation of the optical waveguide. For example, these optical amplifiers can be realised by means of erbium-doped fibre amplifiers or EDFAs in which the amplification is achieved by stimulated emission when the incident light passes through the amplifying medium. Because of a continuous supply of energy, this emission occurs in the form of higher-energy light, that is to say light that is of a shorter wavelength compared to the data signal; this Method is referred to as xe2x80x9coptical pumpingxe2x80x9d. Here the so-called pump light that is produced by a so-called pump laser can be fed to the amplifier via a separate optical waveguide or via the same optical waveguide that is also carrying the data signal. In this case the pump laser can be placed a few kilometers away from the amplifier, which is located at a point under water, for example. As an alternative to a fibre amplifier, amplification can be achieved by means of the so-called
Raman effect, in which amplification is effected by controlled, stimulated scatter in the optical waveguide. For this, pump light is also generated by means of a pump laser, which, however, unlike in the case of the fibre amplifier, is not specifically fed to an optical amplifier, but where the amplification of the data signal is achieved in the optical waveguide itself.
In this case the pumping power of the so-called pump laser can amount to a few watts. Due to the very small diameter of the optical waveguide, this means an enormous concentration of light energy which, if it enters the human eye can cause irreparable damage to eyesight. For safety reasons, in the case of fracture of the optical waveguide, for example damage due to civil engineering works, care must therefore be taken to minimise the risk of personal injury. To this end, a fracture in the optical waveguide in hazardous areas should be reliably detected so that the corresponding pump laser can be immediately shut down.
A known accepted possible solution for detecting a fracture in an optical waveguide consists in installing an optical splitter, constructed as a so-called taper for example, at a specific point in the optical waveguide, via which a small portion of the light energy supplied by the pump laser can be tapped off and fed back to the pump laser via a further dedicated optical waveguide. Despite the supply of light energy, as soon as no energy is fed back a break in the optical waveguide is assumed to have occurred anywhere between the specific point and the pump laser and the pump laser is immediately shut down. However, the implementation of this solution has the disadvantage that, compared to an unmonitored transmission system, an additional optical waveguide is required.
In the following text it is always assumed that the pump laser serves at the same time as the monitoring unit, that is to say the detection of a break in the optical waveguide detects a specific section and effects the shut-down of the pump light. In principle, however, these functions can also be separated, whereby for example, the monitoring unit handles the monitoring of a specific laser path and on detection of a break transmits a shut-down signal to the corresponding pump laser.
The basic idea of the invention is that a monitoring signal emitted or transmitted by a pump laser to the optical waveguide is sent back to the pump laser via the same optical waveguide. To do this, at the point up to which the optical waveguide is to be monitored, in this case also referred to as the monitoring terminal point, a reflection device, for example a so-called Bragg grating, that reflects light energy of a specific wavelength, is placed in the optical waveguide. In addition to the pump light, the pump laser transmits a monitoring signal at this specific wavelength, which differs both from the wavelength of the data signal and from the wavelength of the pump light. As long as the optical waveguide is optically conducting between the pump laser, the pump laser receives the reflected monitoring signal. As soon as there is a break in the optical waveguide, the monitoring signal is usually no longer reflected and the pump laser stops emitting the pump light.
An advantageous development of the invention consists in the pump laser 2 sending out different monitoring signals, of which only the first monitoring signal is reflected at the Bragg grating. In the rare case of a planar cleave this can actually result in total reflection of the signals transmitted by the pump laser. The following different conclusions can be arrived at in the pump laser: if no monitoring signals or both monitoring signals are reflected, then it is concluded that there is a break in the optical waveguide and the pump laser is shut down. If only the first monitoring signal is reflected, then it is assumed that the optical waveguide is intact.
A further advantageous development consists in placing two or more different reflection devices, which reflect different wavelengths, at different points of the optical waveguide. The pump laser emits two or more monitoring signals at different wavelengths, one monitoring signal being reflected at one reflection device, respectively. In the event of a break in the optical waveguide at a specific point, all the monitoring signals whose reflection devices are located after the break are no longer reflected. By evaluating the received monitoring signals, the fracture point can then be accurately located section-by-section.
Further developments of the invention are disclosed in the dependent Claims and in the following description.